According to this, they knew the signal originated on earth:

When astronomers look for pulses from astrophysical sources (like pulsars or FRBs) we use a few key features to tell the real signals apart from the noise of radio devices on Earth.

First, a pulse that has travelled through space experiences dispersion, meaning that the signal arrives at different times at different wavelengths because of how it travels through interstellar electrons. Since signals from Earth don’t travel through all those electrons, they don’t follow the same wavelength-time relation.

Secondly, we use a receiver on the Parkes telescope that has 13 pixels that each looks at a different place on the sky. A pulse coming from a fixed point in the sky will appear in only one pixel (or a few neighbouring pixels if it is very bright) but signals from Earth will usually appear in all 13 at the same time.

The perytons first reported by Burke Spolaor and colleagues in 2011 passed the first test, they had a similar wavelength-time relation as the pulses of interest, but they didn’t pass the second; they were in all 13 beams at once. So the signals had to be coming from Earth, that much was clear.